We provide a consistent way of looking at a range of finite state machines and their algebraic models. Our claim is that the natural representation of transitions of finite state machines is in terms of monoid homomorphisms, and distinct generalisation processes that can be applied to finite state machines correspond to distinct categorical generalisation processes at the level of the algebraic models.

The generalisations we consider are those from deterministic to non-deterministic machines, from one-way to two-way machines, and from read-only machines to read/write machines. Hence the finite state machines we consider, and provide algebraic models for, are (deterministic and non-deterministic) finite state automata, two-way automata, Mealy machines, and bounded Turing machines.

The categorical constructions corresponding to these generalisation processes are, respectively: altering the base category from functions to relations, applying the Geometry of Interaction, or Int construction, and a categorical process, which we refer to as the Comp construction, that uses the tensor on monoidal categories to construct graded categories.